The mobile communications industry is ever demanding for more efficient power amplifiers for mobile base stations, which are also able to operate in a wider frequency band allowing a larger data rate or traffic (i.e. transmit more information per second). For example, one carrier of an older generation mobile WCDMA occupies 4 MHz of bandwidth, while new generations like LTE use 10 MHz and 20 MHz carrier bandwidth. In proposals for future generations of mobile communication technology, the carrier bandwidth is going for 40 MHz up to 100 MHz of digital modulation band per carrier.
Power amplifiers operating as final stages of mobile base station transmitters are the most demanding components due to required output power levels, high power efficiency at Back-Off and high linearity. Among high Efficiency power amplifiers technologies like Linear Amplification with Nonlinear Components (LINC) and Envelope Tracking (ET) using Switch Mode power amplifier (SMPA), the Doherty power amplifier (DPA) is the most mature technology due to its relative simplicity.
One of the most notorious limitation of recent traditional high power Doherty power amplifiers was a narrow frequency operational bandwidth, which was not properly analyzed and frequently mentioned and reported in papers as presumably caused by the impedance inverter or so called Doherty Combiner.
In U.S. Pat. No. 7,078,976, it was demonstrated that the Doherty amplifier relative bandwidth can be expanded up to 30%, and that >3 time load line modulation is possible, if impedance transformation networks in the traditional design of the Doherty amplifier between the output of main amplifying device and the Doherty combiner are excluded from the Doherty amplifier architecture, and a lumped element Doherty combiner made of a chain of low impedance prototype is connected directly to the output of the main device absorbing parasitic output capacitance.
U.S. Pat. No. 7,078,976 also points out that the input network is no less important for wideband operation of Doherty amplifier, and proposes to use a low-pass network to connect the input of the main and peak devices to allow a similar phase frequency response, as well as to use a separate and independent input drive of the Mmin and peak device allowing an arbitrary phase and amplitude control of the input signal for each these devices, which even further improves the operational bandwidth and linearity vs power efficiency.
Another issue for power amplifiers is low stability and/or drops of the power supply voltage at the output terminal of the device, which happen during and after at sharp raises and peaks of the amplified signal envelope. This can be caused by parasitic inductances existing between the active die and the power supply terminal. The result of these events are so termed ‘memory effects’ which require sophisticated digital correction technology.
Traditionally, the power supply of an RF power device on an application board is made of quarter wavelength micro-strip lines connected to the output match structures on the printed circuit board (PCB), introducing typically no less than 7 nH of parasitic inductance for 2 GHz amplifiers. For 1 GHz power amplifiers this inductance becomes even higher, typically >14 nH. The larger this inductance and the modulation frequency in the spectrum of RF envelope of the signal, the stronger the presence of memory effects in the output signal of the device.
U.S. Pat. No. 7,119,623 proposes to resolve this an additional lead through which the supply voltage is directly connected to the internal capacitor which connects a compensation shunt inductance Lsh to ground. U.S. Pat. No. 7,119,623 states that the device design in a standard discrete package with additional leads where an external large capacitance is connected to the die inside the package through a much smaller parasitic inductance of additional leads (<1.5 nH) allows an improvement of the electrical “memory effects” compared to the traditional power supply connection through quarter wave lines.
Additionally, when the shunt inductance Lsh is implemented by bond wires connected between the die and the grounding capacitor, the required distance between the die and the package lead is rather large, around 2 mm. For devices operating at frequencies below 1 GHz, the required shunt inductance Lsh value become so large that two steps or loops of bond wire are needed (as shown in U.S. Pat. No. 7,119,623), which requires an even larger spacing. This creates an undesired parasitic inductance Ld by wires connecting the drain metal bar of the die to the output lead of the package and/or the Doherty power combine network.
The parasitic inductance Ld together with parasitic capacitance of a package lead causes an undesired impedance transformation and introduces an additional phase shift of the signal at the device output, and require an additional 180 degrees of electrical length of the impedance transformer between the Main device and the Doherty combiner, the so called “offset line”. This results in an even larger phase shift which limits the operational bandwidth well below that of the 30% claimed by U.S. Pat. No. 7,078,976. This parasitic inductance is especially harmful in case large dies are used, which even further limits the realization of a wideband high power Doherty power amplifier.
Further, it is to be noted that the output network is not the only important factor for a Doherty amplifier. As realized by the inventors, the proper design of input network is equally important, however, the prior art documents ignore the input network architecture. In fact state of the art Doherty amplifiers with Pout>100 W use LDMOS devices, which exhibit a very low input impedance at a high input Q factor of 6 and 12 at respectively 2 GHz and 1 GHz. The required impedance transformation of input matching networks is therefore much higher than that required for the device output, and the input bandwidth is less than the output bandwidth. As a result, the phase characteristics of the input network of the traditional Doherty power amplifier has a different rate versus frequency than that the output network, which even further affects operational bandwidth.